In recent years, swallowable capsule endoscopes are being developed in a field of endoscope. The capsule endoscope is equipped with an imaging function and a radio communication function. After being swallowed by a patient for an observation of an interior of a body cavity, the capsule endoscope travels through inside organs such as esophagus, stomach, and small intestine following peristaltic movements and sequentially captures images until naturally discharged from a human body.
Image data obtained inside the body cavities by the capsule endoscope while traveling inside the body cavities is sequentially transmitted to an outside of the subject by radio communication, and either accumulated in a memory arranged in a receiver disposed outside the subject, or displayed on a display unit included in the receiver. A doctor and a nurse can make diagnosis based on images presented on the display unit by looking at the images displayed based on the image data accumulated in the memory or by looking at the images displayed on the display unit in the receiver simultaneously with the image data reception.
When the receiver is configured to have a memory for accumulation of the image data, the receiver in general is provided with plural antennas for receiving image signals transmitted from the capsule endoscope. The plural antennas are arranged outside the subject in a dispersed manner, and the receiver selects an antenna whose received strength is high from the plural antennas and receives the image signals by the selected antenna by switching. For example, Patent Document 1 describes a receiver which switches over plural antennas arranged outside the subject and locates the capsule endoscope, i.e., an image signal sender, in the subject based on electric field strength received by each antenna.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-19111